The number seven is the most frequently encountered number in the book of Revelation:

Even the most careless reader of the Apocalypse must be struck by the manner in which almost every thing there is ordered by sevens. Thus, besides the seven Churches, and their seven Angels, we have already in this first chapter the seven Spirits (Rev. Rev. 1:4+), the seven candlesticks (Rev. Rev. 1:12+), the seven stars (Rev. Rev. 1:16+); and further on, the seven lamps of fire (Rev. Rev. 4:4+), seven seals (Rev. Rev. 5:1+), seven horns and seven eyes of the Lamb (Rev. Rev. 5:6+), seven heavenly Angels with their seven trumpets (Rev. Rev. 8:2+), seven thunders (Rev. Rev. 10:3+), seven heads of the dragon, and seven crowns upon these heads (Rev. Rev. 12:13+), the same of the beast rising out of the sea (Rev. Rev. 13:1+), seven last plagues (Rev. Rev. 15:1+;Rev. 1:1+); seven vials (Rev. Rev. 15:7+), seven mountains (Rev. Rev. 17:9+), seven kings (Rev. Rev. 17:10+); not to speak of other recurrences, not so obvious, of this number seven as the signature of the Book; as for instance, the distribution of the entire Book into seven visions, the sevenfold ascription of glory to the Lamb (Rev. Rev. 5:12+), and to God (Rev. Rev. 7:12+).1

As mentioned in our discussion of six, the number seven is understood to denote perfection in the sense of completion. God created in six days and rested on the seventh.6 This is the main symbolism of the number seven in the book of Revelation. The seven churches are representative of all churches. The seven Spirits represent the perfect omniscience of the Holy Spirit.7 The seven seals, trumpets, and bowls denote the completeness of Gods worldwide judgment.8

The prevalence of seven throughout the book of Revelation has also been recognized as signifying this book as the final revelation of God to complete the canon of Scripture: 9

Almost certainly one of the primary reasons [for the preponderance of sevens] is to emphasize that this is the last book of the Bible! In fact, the book closes with a grave warning against anyone who would pretend to add anything further to Gods inspired Word (Revelation Rev. 22:18+).10

It seems likely that John has written his book carefully to signify the perfect plan of God and the completeness of his work.11

With the final acts recorded in the book of Revelation, God completes His mighty act of redemption and renewal thus restoring His creation to the condition it had prior to the entrance of sin.12

12 God completed His mighty work of creating, constructing, and energizing the entire cosmos and all its creatures in the very first seven-day period of history. Because of sin and the curse, He has since been accomplishing His might work of redeeming and saving the creation. One day this work also will be completed.Morris, The Revelation Record, 31.